For example, in various plants, a centrifugal compressor is used to compress a process gas. In the centrifugal compressor, the process gas suctioned into a casing from a suction port is compressed in a flow channel of an impeller by rotation of the impeller together with a rotating shaft, and is discharged outside of the casing from a discharge port.
A discharge volute communicating with the discharge port is formed in the centrifugal compressor casing. The discharge volute keeps the flow velocity of the process gas constant and reduces pressure loss.
Generally, the discharge volute is formed by welding a volute piece, which is a separate member from the casing, to an annular groove formed on an inner surface of the casing (see Patent Literature 1).